Structure-mechanical property relations of non-graphitizing pyrolytic carbon synthesized at low temperatures

Abstract

Polymer derived carbon ceramics are highly desirable for lightweight, high strength, extreme environment material architectures, but their mechanical performance as a function of structure and processing is not currently understood and cannot be predicted. In this study, the mechanical behavior for bulk-scale pyrolytic carbons (PyCs) made via polymer pyrolysis of phenolformaldehyde precursors are established as a function of heat treatment temperature and the resulting average nano-and meso-scale order and disorder via X-ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy. The PyCs exhibit crystallite evolution on both the atomic-and meso-scale for pyrolysis temperatures of 600 • C to 1000 • C, whereas only atomic-scale crystallite evolution is observed for pyrolysis temperatures of 1000 • C to 1400 • C. The measured Vickers hardness of the PyCs is observed to scale non-monotonically as a function of the pyrolysis temperature reaching a peak at ∼ 4 GPa for samples prepared at 1000 • C. New modeling results, based on the elastic constants of disordered graphite, indicate that this counter-intuitive Vickers hardness scaling, which is a decades-old open question, originates from the PyC inter-layer shear elastic constant and the crystallite aspect ratio evolution with processing temperature. PyCs studied here are shown to be the lightest super-hard materials, having Vickers hardness-to-density ratios that are comparable to super-hard carbides, oxides, nitrides, and phosphides.

Evolution of glassy carbon under heat treatment: correlation structure–mechanical properties

“…Further decrease in mechanical performance for higher heat treatment temperatures is related with breaking some of the cross-link bonds during development of more graphitic structures. Similar conclusions were generally given also by other authors in more recent reports [64,66,67].…”

Mesoscale evolution of non-graphitizing pyrolytic carbon in aligned carbon nanotube carbon matrix nanocomposites

“…CNT confinement is found to have a noticeable effect on the crystallite evolution trajectory in PyCs manufactured at T p 1000 • C, however the presence of CNTs has surprisingly little impact on PyCs synthesized at 1000 • C < T p 1400 • C. The x-ray diffraction (XRD) results indicate that aligned CNT PyC matrix nanocomposites (A-CMNCs) with a CNT volume fraction of ∼ 1 vol. % leads the size (L a ) of the graphitic crystallites that comprise the PyC matrix to evolve faster and reach the plateau L a ≈ 5 nm value at T p ∼ 800 • C, while the baseline PyCs achieve L a ≈ 5 nm at a higher T p ∼ 1000 • C, which agrees closely with recent findings on P-F derived PyCs [22]. Additionally, XRD indicates that CNT confinement in the A-CMNCs has little impact on the evolution of the thickness (L c ) and interlayer separation (d 002 ) of the graphitic crystallites that comprise the PyC matrix, which indicates that any CNT templating effects, for example through π − π interactions, primarily impact crystallite growth in the basal plane family of directions.…”

“…S3, Supplementary Materials) and Fig. 2c illustrate, the L a of PyCs grows with increasing T p up to T p ∼ 1000 • C, and that L a plateaus at L a ∼ 5 nm for T p > 1000 • C, which is in good agreement with previous work on P-F undergoing pyrolysis [22]. On the other hand, Fig.…”

“…4 illustrates the presence of C=C stretching in the aromatic rings that comprise the graphitic crystallites, which is represented by the ∼ 1636 cm −1 peak [87,88]. The broadening of the ∼ 1636 cm −1 band could also be attributed to C−O in the benzophenone groups [22]. Having both peaks around ∼ 1374 cm −1 and ∼ 1636 cm −1 could also indicate the presence of C−O stretching originating from carboxyl groups [88].…”

“…1b for their evolution in pure P-F undergoing pyrolysis up to T p ∼ 3000 • C [20][21][22]. Because many P-F resins are commercially available as ultraviolet curable photoresists [55], such as the broadly utilized SU8 photoresist, and typically yield PyCs with low densities ( 1.2 g/cm 3 ) and high hardness ( 4 GPa) [22], they are well-suited for additive manufacturing of next-generation polymer derived ceramic metamaterials. This means that elucidation of the influence of CNT confinement on the nano-, meso-, and microscale evolution of the P-F during pyrolysis at temperatures typical of additive manufacturing processes (i.e.,…”